The present invention relates to semiconductor devices using nitride-based compound semiconductors and, more particularly, to a transistor having a semiconductor/metal/semiconductor structure in which both-side semiconductors and the metal form a Schottky junction. Among such transistors are metal base transistors (MBTs) and permeable base transistors (PBTs)
Studies on MBTs began as early as 1961. The basic band structure is as shown in FIG. 10. This MBT has a structure in which the base is formed of metal with a view to avoiding any increase in base resistance due to reduction of base width, which has been an issue in conventional transistors. In this case, there is a need of forming a Schottky junction between the base metal and both-side semiconductors.
In early studies, there were insufficiencies in techniques for forming semiconductors having enough crystallinity and successful semiconductor/metal interfaces. As a result, it was difficult to form MBTs having successful characteristics (Proceed. IRE 50 (1962) 1534).
Through further studies, MBTs in Si/silicide/Si structures were studied. However, there have been developed no transistors having a large current amplification factor because of pinholes of silicide (Electronics Lett. 20 (1984) 762), (Appl. Phys. Lett. 47 (1985) 151). In addition to this, studies on a combination of ErAs/AlAs (Appl. Phys. Lett. 68 (1996) 84), and a combination of NiAl/GaAs (J.Cryst. Growth 169 (1996) 201) and the like have been made, but successful Schottky characteristics have not been obtained because of poor crystallinity of AlAs or GaAs grown on ErAs or NiAl.
Also, in the PBT, as shown in FIG. 11, a base metal 1 is stripe patterned and, as in the MBT, forms a Schottky junction with an emitter 2 or a collector 3 around the base metal 1 (IEEE Trans. Electron Devices ED-27 (1980) 1128). It is noted that reference numeral 4 denotes an emitter electrode and 5 denotes a collector electrode.
In the conventional PBT, gallium arsenide (GaAs) is used as the semiconductors 2, 3, and tungsten (W) is used as the base metal 1. In observing vicinities of the base metal 1, there can be seen a void 6 where the emitter (GaAs) 2 has not been epitaxially grown on the base metal (W) 1 as shown in FIG. 12. As a result, ideal characteristics have not been obtained.
As the base metal of the MBT or PBT, there have been proposed those using hexaborides of rare earth elements (e.g. LaB6) (Japanese Patent Laid-Open Publication SHO 62-177972), those using metal oxides such as BiRbBaO (Japanese Patent Laid-Open Publication HEI 06-296013), and those using Pt, Pd or Ni (Japanese Patent Laid-Open Publication HEI 06-333938).
As described above, in order to materialize MBTs and PBTs having superior characteristics, the following three issues need to be solved. One of those issues is pinholes that occur to the metal layer. By obtaining a pinhole-free metal layer, all the carriers that are transported from emitter to collector come into hot electrons, allowing transistor characteristics to be enhanced.
Another issue is crystallinity of the semiconductor layer formed on the metal layer. It is relatively easy to grow a metal layer having successful crystallinity on the semiconductor layer having a diamond structure or zinc-blende structure. However, when a semiconductor layer having a diamond structure or zinc-blende structure is grown on the metal layer, there would occur a multiplicity of crystal defects due to antiphase domains for the reason that a polar material is grown on a nonpolar material. As a result, the crystallinity of the semiconductor layer grown on the metal layer would deteriorate to a considerable extent, so that an ideal Schottky junction would not be formed.
The final issue is how to eliminate voids that would occur just above W when the metal (W) is buried into the semiconductor layer. Basically, because the metal (W) is polycrystalline and because the relationship of epitaxial growth is not satisfied between W and semiconductor, there would not be grown a semiconductor layer on the striped metal layer. Whereas semiconductor 2a at portions where the metal layer 1 is absent are grown so as to go around onto the metal layer 1 so that the metal layer 1 is buried as shown in FIGS. 13A-13D, a void 6 to be formed on the metal layer 1 upon contact between the semiconductor 2a, 2a that has gone around from both sides of the metal layer 1 does not completely eliminated. Therefore, in order to make a perfect buried state without the presence of the void 6 on the metal layer 1, the relationship of epitaxial growth needs to be satisfied between the metal layer 1 and the semiconductor layer 2.
Also, in the case where hexaborides of rare earth elements, metal oxides such as BiRbBaO, or metals such as Pt are used as the base material, there would be problems as follows. That is, because crystalline structure (zinc-blende structure or wurtzite structure) and lattice constant of borides or oxides are different from those of general semiconductor, the difference of crystallinity of the emitter layer and the collector layer would be large. As a result, base-emitter electrical characteristics and base-collector electrical characteristics would be largely different from each other.
Accordingly, an object of the present invention is to provide a semiconductor device having successful Schottky characteristics and exhibiting excellent characteristics.
In order to achieve the above object, there is provided a semiconductor device which comprises a three-layer structure composed of a nitride-based compound semiconductor, a metal nitride and a nitride-based compound semiconductor.
In this constitution, since the layers of the three-layer structure of semiconductor/metal/semiconductor are given by nitrides, one identical constituent element is contained in both semiconductor layers and the metal layer, thus making it possible to achieve a spatially continuous crystal growth. Therefore, a semiconductor of good crystallinity is formed, and a successful Schottky junction is obtained. As a result, a semiconductor device superior in electrical characteristics can be obtained.
In one embodiment of the present invention, the metal nitride is formed into a striped shape.
In this constitution, since one identical constituent element is contained in the individual layers of the semiconductor layer/metal layer/semiconductor layer, a spatially continuous crystal growth is enabled so that a semiconductor of good crystallinity can be formed. Therefore, an ideal interface with less voids, as can be seen in GaAs-W, is obtained on the striped metal nitride, and a PBT superior in electrical characteristics can be obtained.
In one embodiment of the present invention, the metal nitride is formed into a meshed shape.
In this constitution, as in the case of striped shape, an ideal interface with less voids is obtained on the meshed metal nitride, and a PBT superior in electrical characteristics can be obtained. Further, in the case where the metal nitride is formed into a meshed shape, even if part of lead-out portion to electrodes is disconnected for some reason, the other of the portion keeps contact, thus eliminating any effects of the disconnection on characteristics. Therefore, there is no need of enlarging the lead-out portion.
In one embodiment of the present invention, metal of the metal nitride is at least one selected from among niobium, tantalum, chromium, zirconium, titanium and vanadium.
In this constitution, because Nb, Ta, Cr, Zr, Ti and V combine with nitrogen to form nitrides at a stoichiometric ratio of 1:1, the resulting metal nitride and the nitride-based compound semiconductor show the same stoichiometric ratio. Therefore, the metal nitride and the nitride-based compound semiconductor can be formed spatially continuously (interface bonding of 1:1), so that an ideal metal/semiconductor interface free from occurrence of dangling bonds and the like can be obtained. As a result, a semiconductor device more superior in electrical characteristics can be obtained.
In one embodiment of the present invention, the nitride-based compound semiconductor is AlGaInN with lanthanoid added thereto.
In this constitution, since the lattice constant can arbitrarily be changed by adding lanthanoid to AlGaInN, the lattice matching between the nitride-based compound semiconductor and the metal nitride becomes controllable. Therefore, crystallinity of the nitride-based compound semiconductor is improved, and a semiconductor device more superior in electrical characteristics can be obtained.
In one embodiment of the present invention, the lanthanoid-added AlGaInN and the metal nitride are formed in such a way that a (0001) plane of the lanthanoid-added AlGaInN and a (111) plane of the metal nitride are parallel to each other.
In this constitution, the (0001) plane (c plane) of the lanthanoid-added AlGaInN whose crystal structure is a wurtzite structure, and the (111) plane of the metal nitride whose crystal structure is an halitic (lock solt) structure become the same close-packed planar structure so that the epitaxial relationship is satisfied. Therefore, by such a crystal growth that the (0001) plane of the lanthanoid-added AlGaInN and the (111) plane of the metal nitride become parallel to each other, a successful crystallinity can be obtained.
Particularly in the case of the PBT, since the epitaxial relationship between the metal nitride and the lanthanoid-added AlGaInN is satisfied, the adhesion of the metal nitride with the lanthanoid-added AlGaInN provided thereon is improved, so that an ideal interface having almost no voids can be obtained.
In one embodiment of the present invention, the nitride-based compound semiconductor is a nitride of lanthanoid.
In this constitution, with an halitic structure adopted, when the nitride-based compound semiconductor is given by a lanthanoid nitride whose orientation of growth is unselected so as to be of the same crystal structure as the metal nitride, it becomes possible to obtain a crystallinity successful in every orientation.
In one embodiment of the present invention, the lanthanoid is at least one selected from among lanthanum, cerium, praseodymium, neodymium, prometium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium and lutetium.
In this constitution, when Gd, Tb, Dy, Ho or Er, which is more stable in trivalent state, is added to AlGaInN, a more stable lanthanoid-added AlGaInN can be obtained. Also, even with any lanthanoid for use as a nitride, a crystallinity successful in every orientation can be obtained. Further, since the lanthanoid is a high melting point metal, the resulting nitride-based compound semiconductor itself becomes more stable to temperature.